1. Field of the Invention
The present invention relates to a fuel cell having a membrane electrode assembly comprising a solid polymer ion exchange membrane, an anode electrode mounted on one surface of the solid polymer ion exchange membrane, and a cathode electrode mounted on the other surface of the solid polymer ion exchange membrane, the membrane electrode assembly being sandwiched between an anode separator and a cathode separator, and a method of operating such a fuel cell.
2. Description of the Related Art
Usually, solid polymer electrolyte fuel cells employ an ion exchange membrane comprising a polymer ion exchange membrane (cation exchange membrane). A solid polymer electrolyte fuel cell comprises a unit cell (unit generation cell) comprising a joint body (membrane electrode assembly) made up of anode and cathode electrodes, each having a base made chiefly of carbon and a catalytic electrode layer of precious metal, disposed respectively on both sides of an ion exchange membrane, the joint body being sandwiched between separators (bipolar plates). Usually, a certain number of such unit cells are stacked to form a fuel cell stack.
In the fuel cell of the above type, a fuel gas such as a gas mainly containing hydrogen (hereinafter referred to as “hydrogen-containing gas”) is supplied to the anode electrode, and hydrogen is ionized on the catalytic electrode layer of the anode electrode and moves through the ion exchange membrane toward the cathode electrode. Electrons produced while the hydrogen ions are moving toward the cathode electrode are extracted by an external circuit and used as DC electric energy.
Since the cathode electrode is supplied with an oxidizing gas, e.g., a gas mainly containing oxygen or air (hereinafter referred to as “oxygen-containing gas”), the hydrogen ions, the electrons, and the oxygen react with each other, producing water (hereinafter also referred to as “reaction-produced water”) at the cathode electrode.
The fuel cell stack has been disadvantageous in that because water produced by an electrochemical reaction tends to be accumulated at the cathode electrode, the fuel cell stack is liable to have its electric generating capability lowered and the membrane electrode assembly is likely to be unduly expanded. Various attempts have heretofore been made to remove the water accumulated at the cathode electrode.
For example, there are known a technique disclosed in U.S. Pat. No. 5,260,143 (hereinafter referred to as “first prior art”), a technique disclosed in U.S. Pat. No. 5,441,819 (hereinafter referred to as “second prior art”), and a technique disclosed in U.S. Pat. No. 5,547,776 (hereinafter referred to as “third prior art”).
According to the first prior art, a pressure drop is provided between the inlet and outlet of a cathode gas passage to remove reaction-produced water from the cathode. Specifically, the pressure drop is developed by providing an orifice at the inlet of the cathode gas passage, elongating the cathode gas passage, or changing the cross-sectional shape of the cathode gas passage.
According to the second prior art, the temperature of a hydrogen-containing gas supplied from the inlet to outlet of an anode gas passage is kept at a temperature equal to or higher than the condensation temperature of water vapor contained in the hydrogen-containing gas. Water accumulated at the cathode is diffused back toward the anode due to a concentration gradient, and removed as water vapor into the hydrogen-containing gas.
According to the third prior art, a temperature gradient is provided in the plane of a cathode electrode to establish a low-temperature area corresponding to an area where an oxygen-containing gas contains a minimum amount of water and a high-temperature area corresponding to an area where the oxygen-containing gas contains a maximum amount of water.
The first prior art is problematic in that since the pressure drop is provided in the cathode gas passage, the supplied amount of reactive gases (the hydrogen-containing gas and the oxygen-containing gas) is limited, resulting in a reduction in the electric generating efficiency.
The second prior art is disadvantageous in that a temperature control process for controlling the temperature of the hydrogen-containing gas at a desired temperature is considerably complex, and cannot be performed highly accurately.
Problems of the third prior art are that the temperature gradient provided in the plane of the cathode electrode limits the shape of a gas passage, reducing the freedom of design, and dimensional changes caused by thermal expansion develop clearances or gaps in the fuel cell stack, causing a reduction in the electric generating capability thereof.
The inventions according to the first through third prior art are aimed at only the removal of water accumulated at the cathode. However, water is also accumulated at the anode because the relative humidity of the water vapor increases when the hydrogen contained in the hydrogen-containing gas is consumed. The water accumulated at the anode needs to be removed in order to keep the electric generating capability at an effective level.